Mysterious Giant “Bubbles” Discovered at Center of Milky Way

(THIS ARTICLE IS COURTESY OF SCIENCE TECH DAILY)

 

Mysterious Giant “Bubbles” Discovered at Center of Milky Way [Video]

Radio Image of the Central Portions of the Milky Way Galaxy

International team detected radio bubbles with South Africa’s MeerKAT telescope.

A gigantic, balloon-like structure has been hiding in plain sight, right in the center of our own galaxy.

An international team of astronomers, including Northwestern’s Farhad Yusef-Zadeh, discovered the structure, which is one of the largest ever observed in the Milky Way’s center. The newly spotted pair of radio-emitting bubbles reach hundreds of light-years tall, dwarfing all other structures in the central region of the galaxy.

The team believes the enormous, hourglass-shaped structure likely is the result of a phenomenally energetic burst that erupted near the Milky Way’s super massive black hole several million years ago.

“The center of our galaxy is relatively calm when compared to other galaxies with very active central black holes,” said Ian Heywood of the University of Oxford, first author of study. “Even so, the Milky Way’s central black hole can — from time to time — become uncharacteristically active, flaring up as it periodically devours massive clumps of dust and gas. It’s possible that one such feeding frenzy triggered powerful outbursts that inflated this previously unseen feature.”

Using MeerKAT

Why couldn’t we see such a massive figure before? We simply did not have the technology. Until now, the enormous bubbles were hidden by extremely bright radio emissions from the center of the galaxy. For this work, the team used the South African Radio Astronomy Observatory (SARAO) MeerKAT telescope, the largest science project in Africa. The radio light seen by MeerKAT can easily penetrate the dense clouds of dust that block visible light from the center of the galaxy.

This is the first paper detailing research completed with MeerKAT’s full 64-dish array since its launch in July 2018.

South African MeerKAT Radio Telescope

More turbulent and unusually active compared to rest of the Milky Way, the environment surrounding our galaxy’s central black hole holds many mysteries. Northwestern’s Yusef-Zadeh, a senior author of the paper, has dedicated his career to studying the physical processes that occur in the Milky Way’s mystifying center.

In the early 1980s, Yusef-Zadeh discovered large-scale, highly organized magnetic filaments in the center of the Milky Way, 25,000 light-years from Earth. While their origin has remained an unsolved mystery ever since, the filaments are radio structures stretching tens of light-years long and one light-year wide.

“The radio bubbles discovered with MeerKAT now shed light on the origin of the filaments,” Yusef-Zadeh said. “Almost all of the more than 100 filaments are confined by the radio bubbles.”

Researchers believe the close association of the filaments with the bubbles implies that the energetic event that created the radio bubbles also is responsible for accelerating the electrons required to produce the radio emission from the magnetized filaments.

The team of astronomers on this project represents 15 institutions, including Northwestern, Oxford, the South African Radio Astronomy Observatory in Cape Town and the National Radio Astronomy Observatory in Virginia.

The research paper appears in the journal Nature.

For more on this discovery, read Staggeringly Powerful Event Occurred Near Center of the Milky Way.

Reference: “Inflation of 430-parsec bipolar radio bubbles in the Galactic Centre by an energetic event” by I. Heywood, F. Camilo, W. D. Cotton, F. Yusef-Zadeh, T. D. Abbott, R. M. Adam, M. A. Aldera, E. F. Bauermeister, R. S. Booth, A. G. Botha, D. H. Botha, L. R. S. Brederode, Z. B. Brits, S. J. Buchner, J. P. Burger, J. M. Chalmers, T. Cheetham, D. de Villiers, M. A. Dikgale-Mahlakoana, L. J. du Toit, S. W. P. Esterhuyse, B. L. Fanaroff, A. R. Foley, D. J. Fourie, R. R. G. Gamatham, S. Goedhart, S. Gounden, M. J. Hlakola, C. J. Hoek, A. Hokwana, D. M. Horn, J. M. G. Horrell, B. Hugo, A. R. Isaacson, J. L. Jonas, J. D. B. L. Jordaan, A. F. Joubert, G. I. G. Józsa, R. P. M. Julie, F. B. Kapp, J. S. Kenyon, P. P. A. Kotzé, H. Kriel, T. W. Kusel, R. Lehmensiek, D. Liebenberg, A. Loots, R. T. Lord, B. M. Lunsky, P. S. Macfarlane, L. G. Magnus, C. M. Magozore, O. Mahgoub, J. P. L. Main, J. A. Malan, R. D. Malgas, J. R. Manley, M. D. J. Maree, B. Merry, R. Millenaar, N. Mnyandu, I. P. T. Moeng, T. E. Monama, M. C. Mphego, W. S. New, B. Ngcebetsha, N. Oozeer, A. J. Otto, S. S. Passmoor, A. A. Patel, A. Peens-Hough, S. J. Perkins, S. M. Ratcliffe, R. Renil, A. Rust, S. Salie, L. C. Schwardt, M. Serylak, R. Siebrits, S. K. Sirothia, O. M. Smirnov, L. Sofeya, P. S. Swart, C. Tasse, D. T. Taylor, I. P. Theron, K. Thorat, A. J. Tiplady, S. Tshongweni, T. J. van Balla, A. van der Byl, C. van der Merwe, C. L. van Dyk, R. Van Rooyen, V. Van Tonder, R. Van Wyk, B. H. Wallace, M. G. Welz and L. P. Williams, 11 September 2019, Nature.
DOI: 10.1038/s41586-019-1532-5

NASA shock: The most massive object in the universe is forming before our very own eyes

(THIS ARTICLE IS COURTESY OF THE UK EXPRESS NEWS)

 

NASA shock: The most massive object in the universe is forming before our very own eyes

NASA astronomers have detected the merger of four galactic clusters in deep space will give birth to one of the most massive objects in the universe.

NASA supercomputer: A trip through the ‘universe machine’

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 estimates the resulting mega-cluster will combine weights “several hundred trillion times” more than our Sun. The monstrous body will emerge from the collision of four galactic clusters about three billion light-years from Earth. Each cluster carries hundreds or thousands of galaxies of its own, suspended in a cloud of hot gas.

Astronomers have dubbed the slowly emerging “mega-structure” Abell 1758.

NASA estimates the individual bodies in the collisions are moving at speeds of two to three million miles per hour.

The incredible discovery was made possible thanks to NASA’s Chandra X-ray Observatory  probe.

NASA said: “Astronomers using data from the Chandra X-ray Observatory and other telescopes have put together a detailed map of a rare collision between four galaxy clusters.

READ MORE: 

NASA news: Four galactic clusters merging into one

NASA news: These four galactic clusters are merging into a mega-cluster (Image: NASA NASA/CXC/SAO/G.Schellenberger et al.;)

NASA news: Chandra X-ray Observatory

NASA news: The Chandra Observatory watches the universe in X-ray wavelenghts (Image: NASA/CXC & J VAUGHAN)

“Eventually, all four clusters – each with a mass of at least several hundred trillion times that of the Sun – will merge to form one of the most massive objects in the universe.”

Galactic clusters are large groupings of individual galaxies bound together by their collective gravities and the gravity of dark matter.

The gravitational attraction also explains why clusters are suspended in a cloud of gas.

Galaxies will typically expel their stellar gases when stars erupt into supernovas.

Clusters hang onto these gases and are some of the largest known objects in the universe.

In the particular case of Abell 1758, astronomers have detected four separate bodies colliding into two larger clusters.

All four clusters will merge to form one of the most massive objects in the universe

NASA

In time, the resulting two bodies will fall towards one another to produce an even bigger mega-structure.

NASA said: “Sometimes two galaxy clusters collide, as in the case of the Bullet Cluster, and occasionally more than two will collide at the same time.

“The new observations show a mega-structure being assembled in a system called Abell 1758, located about three billion light-years from Earth.

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NASA news: Percentage of NASA's budget over years

NASA news: The percentage of NASA’s budget over the years (Image: EXPRESS)

NASA news: Galactic cluster in deep space

NASA news: Galactic clusters are large groups of galaxies suspended in a cloud of gas (Image: NASA)

“It contains two pairs of colliding galaxy clusters that are heading toward one another.

“Scientists first recognised Abell 1758 as a quadruple galaxy cluster system in 2004 using data from Chandra and XMM-Newton, a satellite operated by the European Space Agency (ESA).”

Chandra’s Observatory charts the universe in X-ray wavelengths instead of visible light.

In the above picture, the X-rays are seen in blue and white light.

NASA: Three black holes heading towards a merger

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What happens when galaxies collide?

Galactic collisions are a fairly frequent occurrence in deep space and our Milky Way is no exception.

Approximately four billion years from now the neighboring Andromeda galaxy will crash into the Milky Way.

Thankfully, the space in-between individual planets and stars is too great for any bodies to collide.

But the collisions will have a profound effect on the shape and movement of the galaxies.

When two spiral galaxies collide, for instance, they can end up creating an elliptical galaxy.

The galactic merger can also trigger the birth of new stars thanks to stellar gas and dust mixing in the presence of gravity.

Quick facts about NASA’s Chandra X-ray Observatory

1. NASA’s Chandra is the world’s most powerful X-ray observatory.

2. The space telescope was launched into orbit by a space shuttle.

3. Chandra can resolve the individual letters of a stop sign from 12 miles away.

4. The space telescope orbits the Earth 200 times higher than the Hubble Space Telescope.

5. Thanks to its X-ray capabilities, Chandra can peer through clouds of stellar gas that otherwise obscure hidden bodies.

Astronomers Create 8 Million Baby Universes Inside A Computer

(THIS ARTICLE IS COURTESY OF LIVE SCIENCE)

 

Astronomers Create 8 Million Baby Universes Inside a Computer and Watch Them Grow. Here’s What They Learned.

helix nebula

(Image: © Shutterstock)

A team of astrophysicists has just spawned 8 million unique universes inside a supercomputer and let them evolve from just tots to old geezers. Their goal? To nail down the role that an invisible substance called dark matter played in our universe’s life since the Big Bang and what it means for our fate.

After discovering that our universe is mostly composed of dark matter in the late 1960s, scientists have speculated on its role in the formation of galaxies and their ability to give birth to new stars over time.

According to the Big Bang theory, not long after the universe was born, an invisible and elusive substance physicists have dubbed dark matter began to clump together by the force of gravity into massive clouds called dark matter haloes. As the haloes grew in size, they attracted the sparse hydrogen gas permeating the universe to come together and form the stars and galaxies we see today. In this theory, dark matter acts as the backbone of galaxies, dictating how they form, merge and evolve over time.

Related: The 11 Biggest Unanswered Questions About Dark Matter

To better understand how dark matter shaped this history of the universe, Peter Behroozi, an assistant professor of astronomy at the University of Arizona, and his team created his own universes using the school’s supercomputer. The computer’s 2,000 processors worked without pause over a span of three weeks to simulate more than 8 million unique universes. Each universe individually obeyed a unique set of rules to help researchers understand the relationship between dark matter and the evolution of galaxies.

“On the computer, we can create many different universes and compare them to the actual one, and that lets us infer which rules lead to the one we see,” Behroozi said in a statement.

While previous simulations have focused on modeling single galaxies or generating mock universes with limited parameters, the UniverseMachine is the first of its scope. The program continuously created millions of universes, each containing 12 million galaxies, and each allowed to evolve over nearly the entire history of the real universe from 400 million years after the Big Bang to the present day.

“The big question is, ‘How do galaxies form?’” said study researcher Risa Wechsler, a professor of physics and astrophysics at Stanford University. “The really cool thing about this study is that we can use all the data we have about galaxy evolution —  the numbers of galaxies, how many stars they have and how they form those stars — and put that together into a comprehensive picture of the last 13 billion years of the universe.”

Related: From the Big Bang to Present: Snapshots of Our Universe Through Time

Creating a replica of our universe, or even of a galaxy, would require an inexplicable amount of computing power. So Behroozi and his colleagues narrowed their focus to two key properties of galaxies: their combined mass of stars and the rate at which they give birth to new ones.

“Simulating a single galaxy requires 10 to the 48th computing operations,” Behroozi explained, referring to an octillion operation, or a 1 followed by 48 zeros. “All computers on Earth combined could not do this in a hundred years. So to just simulate a single galaxy, let alone 12 million, we had to do this differently.”

As the computer program spawns new universes, it makes a guess on how a galaxy’s rate of star formation is related to its age, its past interactions with other galaxies and the amount of dark matter in its halo. It then compares each universe with real observations, fine-tuning the physical parameters with every iteration to better match reality. The end result is a universe nearly identical to our own.

According to Wechsler, their results showed that the rate at which galaxies give birth to stars is tightly connected to the mass of their dark matter haloes. Galaxies with dark matter halo masses most similar to our own Milky Way had the highest star-formation rates. She explained that star formation is stifled in more massive galaxies by an abundance of blackholes

Their observations also challenged long-held beliefs that dark matter stifled star formation in the early universe.

“As we go back earlier and earlier in the universe, we would expect the dark matter to be denser, and therefore the gas to be getting hotter and hotter. This is bad for star formation, so we had thought that many galaxies in the early universe should have stopped forming stars a long time ago,” Behroozi said. “But we found the opposite: Galaxies of a given size were more likely to form stars at a higher rate, contrary to the expectation.”

Now, the team plans to expand the Universe Machine to test more ways dark matter might affect the properties of galaxies, including how their shapes evolve, the mass of their black holes and how often their stars go supernova.

“For me, the most exciting thing is that we now have a model where we can start to ask all of these questions in a framework that works,” Wechsler said. “We have a model that is inexpensive enough computationally, that we can essentially calculate an entire universe in about a second. Then we can afford to do that millions of times and explore all of the parameter space.”

The research group published their results in the September issue of the journal Monthly Notices of the Royal Astronomical Society.

Originally published on Live Science.

Astrophysicists announce discovery that could rewrite story of how galaxies die

(THIS ARTICLE IS COURTESY OF PHYSICS.ORG)

 

Astrophysicists announce discovery that could rewrite story of how galaxies die

Astrophysicist announces her discovery that could rewrite story of how galaxies die
This artist conception depicts an energetic quasar which has cleared the center of the galaxy of gas and dust, and these winds are now propagating to the outskirts. Soon, there will be no gas and dust left, and only a luminous blue quasar will remain. Credit: Michelle Vigeant

At the annual meeting of the American Astronomical Society in St. Louis, Missouri, Allison Kirkpatrick, assistant professor of physics and astronomy at the University of Kansas, will announce her discovery of “cold quasars”—galaxies featuring an abundance of cold gas that still can produce new stars despite having a quasar at the center—a breakthrough finding that overturns assumptions about the maturation of galaxies and may represent a phase of every galaxy’s lifecycle that was unknown until now.

Her news briefing, entitled “A New Population of Cold Quasars,” takes place Wednesday, June 12, on the 2nd floor of the St. Louis Union Station Hotel.

A quasar, or “quasi-stellar radio source,” is essentially a  on steroids. Gas falling toward a quasar at the center of a galaxy forms an “accretion disk” which can cast off a mind-boggling amount of electromagnetic energy, often featuring luminosity hundreds of times greater than a typical galaxy. Typically, formation of a quasar is akin to galactic retirement, and it’s long been thought to signal an end to a galaxy’s ability to produce .

“All the gas that is accreting on the black hole is being heated and giving off X-rays,” Kirkpatrick said. “The wavelength of light that you give off directly corresponds to how hot you are. For example, you and I give off infrared light. But something that’s giving off X-rays is one of the hottest things in the universe. This gas starts accreting onto the black hole and starts moving at relativistic speeds; you also have a magnetic field around this gas, and it can get twisted up. In the same way that you get solar flares, you can have jets of material go up through these magnetic field lines and be shot away from the black hole. These jets essentially choke off the gas supply of the galaxy, so no more gas can fall on to the galaxy and form new stars. After a galaxy has stopped forming stars, we say it’s a passive dead galaxy.”

But in Kirkpatrick’s survey, about 10 percent of  hosting accreting supermassive  had a supply of cold gas remaining after entering this phase, and still made new .

Astrophysicist announces her discovery that could rewrite story of how galaxies die
An optical blue quasar at a lookback time of 7 billion years (this is not a nearby galaxy). Normally, something like this would not have infrared emission. Credit: Dark Energy Camera Legacy Survey DR7/NOAO

“That in itself is surprising,” she said. “This whole population is a whole bunch of different objects. Some of the galaxies have very obvious merger signatures; some of them look a lot like the Milky Way and have very obvious spiral arms. Some of them are very compact. From this diverse population, we then have a further 10 percent that is really unique and unexpected. These are very compact, blue, luminous sources. They look exactly like you would expect a supermassive black hole to look in the end stages after it has quenched all of the star formation in a galaxy. This is evolving into a passive elliptical galaxy, yet we have found a lot of cold gas in these as well. These are the population that I’m calling ‘cold quasars.'”

The KU astrophysicist suspected the “cold quasars” in her survey represented a brief period yet to be recognized in the end-phases of a galaxy’s lifespan—in terms of a human life, the fleeting “cold quasar” phase may something akin to a galaxy’s retirement party.

“These galaxies are rare because they’re in a transition phase—we’ve caught them right before star formation in the galaxy is quenched and this transition period should be very short,” she said.

Kirkpatrick first identified the objects of interest in an area of the Sloan Digital Sky Survey, the most detailed digital map of the universe available. In an area dubbed “Stripe 82,” Kirkpatrick and her colleagues were able to visually identify quasars.

“Then we went over this area with the XMM Newton telescope and surveyed it in the X-ray,” she said. “X-rays are the key signature of growing black holes. From there, we surveyed it with the Herschel Space Telescope, a far infrared telescope, which can detect dust and gas in the host galaxy. We selected the galaxies that we could find in both the X-ray and in the infrared.”

Astrophysicist announces her discovery that could rewrite story of how galaxies die
The dust emission of the same blue-quasar galaxy. It is surprisingly bright — in fact, it’s one of the brightest objects in the field, indicating a lot of dust. Due to the resolution of the telescope, we cannot see what that dust actually looks like. Credit: Herschel/ESA

The KU researcher said her findings give scientists new understanding and detail of how the quenching of star formation in galaxies proceeds, and overturns presumptions about quasars.

“We already knew quasars go through a dust-obscured phase,” Kirkpatrick said. “We knew they go through a heavily shrouded phase where dust is surrounding the supermassive black hole. We call that the red quasar phase. But now, we’ve found this unique transition regime that we didn’t know before. Before, if you told someone you had found a luminous quasar that had a blue optical color—but it still had a lot of dust and gas in it, and a lot of star formation—people would say, ‘No, that’s not the way these things should look.'”

Next, Kirkpatrick hopes to determine if the “cold quasar” phase happens to a specific class of galaxies or every galaxy.

“We thought the way these things proceed was you have a growing black hole, it’s enshrouded by dust and gas, it begins to blow that material out,” she said. “Then it becomes a luminous blue object. We assumed when it blew out its own gas, it would blow out its host gas as well. But it seems with these objects, that’s not the case. These have blown out their own dust—so we see it as a blue object—but they haven’t yet blown out all of the dust and gas in the host galaxies. This is a transition phase, let’s say of 10 million years. In universal timescales, that’s really short—and it’s hard to catch this thing. We’re doing what we call a blind survey to find objects we weren’t looking for. And by finding these objects, yes, it could imply that this happens to every galaxy.”

“One Trillion Times Age Of The Cosmos”–Rarest Thing Ever Detected

(THIS ARTICLE IS COURTESY OF THE ‘DAILY GALAXY’)

 

“One Trillion Times Age of the Cosmos” –Rarest Thing Ever Detected

 

Cluster Abell 3827

 

“We actually saw this decay happen. It’s the longest, slowest process that has ever been directly observed, and our dark matter detector was sensitive enough to measure it,” said Ethan Brown, an assistant professor of physics at Rensselaer Polytechnic Institute. “It’s an amazing to have witnessed this process, and it says that our detector can measure the rarest thing ever recorded.”

How do you observe a process that takes more than one trillion times longer than the age of the universe? The XENON Collaboration research team did it with an instrument built to find the most elusive particle in the universe—dark matter. In a paper to be published tomorrow in the journal Nature, researchers announce that they have observed the radioactive decay of xenon-124, which has a half-life of 1.8 X 1022 years.

The XENON Collaboration runs XENON1T, a 1,300-kilogram vat of super-pure liquid xenon shielded from cosmic rays in a cryostat submerged in water deep 1,500 meters beneath the Gran Sasso mountains of Italy. The researchers search for dark matter by recording tiny flashes of light created when particles interact with xenon inside the detector. And while XENON1T was built to capture the interaction between a dark matter particle and the nucleus of a xenon atom, the detector actually picks up signals from any interactions with the xenon.

Dark Matter –“Emerged From an Eon Before the Big Bang” (Weekend Feature)

The evidence for xenon decay was produced as a proton inside the nucleus of a xenon atom converted into a neutron. In most elements subject to decay, that happens when one electron is pulled into the nucleus. But a proton in a xenon atom must absorb two electrons to convert into a neutron, an event called “double-electron capture.”

Double-electron capture only happens when two of the electrons are right next to the nucleus at just the right time, Brown said, which is “a rare thing multiplied by another rare thing, making it ultra-rare.”

When the ultra-rare happened, and a double-electron capture occurred inside the detector, instruments picked up the signal of electrons in the atom re-arranging to fill in for the two that were absorbed into the nucleus.

“Ultralight” –‘Dark Matter Exists Beyond the Standard Model’

“Electrons in double-capture are removed from the innermost shell around the nucleus, and that creates room in that shell,” said Brown. “The remaining electrons collapse to the ground state, and we saw this collapse process in our detector.”

The achievement is the first time scientists have measured the half-life of this xenon isotope based on a direct observation of its radioactive decay.

“This is a fascinating finding that advances the frontiers of knowledge about the most fundamental characteristics of matter,” said Curt Breneman, dean of the School of Science. “Dr. Brown’s work in calibrating the detector and ensuring that the xenon is scrubbed to the highest possible standard of purity was critical to making this important observation.”

Very Weird Galaxies –“The Absence of Dark Matter is Unprecedented”

The XENON Collaboration includes more than 160 scientists from Europe, the United States, and the Middle East, and, since 2002, has operated three successively more sensitive liquid xenon detectors in the Gran Sasso National Laboratory in Italy. XENON1T, the largest detector of its type ever built, acquired data from 2016 until December 2018, when it was switched off. Scientists are currently upgrading the experiment for the new XENONnT phase, which will feature an active detector mass three times larger than XENON1T. Together with a reduced background level, this will boost the detector’s sensitivity by an order of magnitude.

Three years ago researchers were excited to find that a galaxy at the heart of cluster Abell 3827 shown at the top of the page that appeared to have separated from the dark matter that surrounded it. New research suggests this is incorrect. (Nasa/ESA/Richard Massey)

The Daily Galaxy via Rensselaer Polytechnic Institute

Dark Matter And Black Holes

(THIS ARTICLE IS COURTESY OF ‘WIRED’ NEWS)

 

WHEN IT COMES to the nature of dark matter, astronomers are still largely, well, in the dark. The existence of this mysterious substance was hypothesized more than 40 years ago to explain discrepancies between the calculations of how galaxies ought to behave, based on their mass, and what was actually observed. In short, it seemed like mass was missing. So Vera Rubin, the astronomer who first discovered this discrepancy, conjured an invisible substance that is far more abundant than “normal” matter and acts as the scaffolding for the large-scale structure of the universe. Today we call it dark matter.

Yet decades of hunting for the elusive dark matter particle still have not yielded direct evidence of its existence. Most cosmologists still believe that dark matter must exist, but some have splintered off to propose other explanations that explain away dark matter by modifying our understanding of gravity.

But two findings are now casting doubt on the modified gravity explanation. In March, a team of astronomers led by Yale professor Pieter van Dokkum and his graduate student Shany Danieli published two papers, one confirming the existence of a galaxy that appears to have almost no dark matter and the other announcing the discovery of a second galaxy of this type. The irony, the researchers say, is that the seeming lack of dark matter in these galaxies is strong evidence that it exists.

The reason they believe these galaxies have no dark matter is that their dynamics can be predicted using our traditional theories of gravity. The discrepancy of the “missing mass” that’s seen in most galaxies isn’t present here, meaning there’s no need for dark matter to explain their behavior. And it means that the modified version of gravity proposed by some cosmologists doesn’t predict these galaxies’ movements as cleanly as good old Newtonian physics.

The discovery of these dark-matter-free galaxies traces back to 2014, when van Dokkum and his colleagues finished building Dragonfly, a new kind of telescope, made of off-the-shelf telephoto camera lenses, that specializes in observing extremely faint celestial objects. Only a year after its first light, Dragonfly discovered a new galaxy characterized by an extreme lack of stars relative to its size. Known as an ultra-diffuse galaxy, this ghostly celestial object had roughly the same mass as our Milky Way, but only one hundredth of one percent of that mass could be attributed to “normal” matter like stars. In other words, van Dokkum and his colleagues had discovered a galaxy made of 99.99 percent dark matter.

While this galaxy was unique, its existence isn’t entirely surprising. Most cosmologists think that dense collections of dark matter act as a sort of seed for the formation of large celestial objects like galaxies. The general idea, says Anže Slosar, an astrophysicist at Brookhaven National Laboratory, is that once a collection of dark matter reaches a critical density, it collapses under its own gravity and forms a so-called “dark matter halo.” This halo, in turn, gravitationally attracts hydrogen gas to its center, where it begins to form stars and, eventually, galaxies. The mass of a dark matter halo varies from galaxy to galaxy, but it seemed like every galaxy must have at least some dark matter to keep its form. Indeed, this assumption was precisely what made Dragonfly’s next discovery so surprising.

In 2016, van Dokkum and his colleagues at Yale discovered NGC 1052-DF2, an ultra-diffuse galaxy that appeared to contain little to no dark matter at all. Last year, when the Yale astronomers published their results in the journal Nature, their peers in the cosmological community were incredulous. This was the first galaxy ever discovered that appeared to lack any dark matter, and as Carl Sagan rightly observed, “extraordinary claims require extraordinary evidence”—which is what many cosmologists thought the Yale team was missing.

University of Pennsylvania astrophysicist Robyn Sanderson says the skepticism about DF2 sprang mostly from the limited amount of data used to draw the conclusion. In this case, the Yale team was using data from just 10 star clusters observed over a period of two nights. This meant it was possible they were overlooking key details of the star clusters’ motion, which would distort their estimations of the galaxy’s mass—and undermine their claim that it lacked dark matter.

The Yale researchers recognized this possible source of error themselves when they published their paper on DF2. The only way to resolve this conundrum was to make more detailed measurements or to find another galaxy with characteristics similar to DF2. In March, the Yale team published two papers that did exactly these things.

The first paper offered more refined measurements of stellar velocities within DF2. This time, rather than just measuring the velocities of 10 star clusters, van Dokkum and Danieli used the Keck telescope in Hawaii to observe the velocities of the stars within the star clusters. This approach produced far more data that reinforced the team’s earlier conclusion that the galaxy lacked dark matter.

The other paper announced the discovery of a second galaxy, DF4, which also appears to have little, if any, dark matter. Not only does this increase the odds that the DF2 observations are accurate, it also means such ultra-diffuse galaxies might not be so rare. The fact that two were found in quick succession, Danieli says, was “really reassuring.” Nevertheless, she says “it’s still too early to say whether they are super rare or quite common.” The team will begin observing other nearby ultra-diffuse galaxies next month in an effort to answer this question.

But that won’t resolve the mystery of how these strange galaxies came to exist in the first place. Theoretical cosmologists will have to run simulations to determine how a galaxy can lose its dark matter, she says. One leading theory involves tidal interactions, which is astronomer-speak for when the gravitational forces of two neighboring galaxies pull material from each galaxy and distort them. DF2 and DF4 are both near the galaxy NGC 1052, which makes it a strong candidate for the galaxy that stole their dark matter.

However they came to be, Danieli argues that the existence of these galaxies is a blow to the modified gravityexplanation for why most galaxies don’t behave as we’d expect.

Known as modified Newtonian dynamics, or MOND, this theory recasts gravity such that it has different effects at the galactic scale. Although MOND has successfully predicted the stellar dynamics of hundreds of galaxies, most of which are relatively isolated, it must be able to predict the dynamics of all galaxies to dethrone dark matter as the going cosmological theory.

As Slosar explains it, the discovery of DF2 and DF4 strengthens the case for the existence of a dark matter particle because it means that it can be separated from normal matter. Because these galaxies behave in line with standard gravitational theory, using the equations discovered by Newton and Kepler, they present a challenge to MOND.

“If you find galaxies, some of which have a lot of dark matter and some of which have a little dark matter, you can’t explain it with the loss of gravity unless you’re willing to say that one part of the universe has a different law of gravity than another part, which is just silly,” Slosar says. “The entire point of physics is to find unified laws that are always there. This is why it is an argument for the existence of dark matter.”

So does the existence of galaxies devoid of dark matter pose an existential threat for MOND? Stacy McGaugh, an astronomer at Case Western Reserve University, doesn’t think so. “When DF2 was first discovered, it was portrayed as a huge problem for MOND,” McGaugh says. “On more careful analysis, it turned out that the prediction of MOND was spot-on what was observed.”

The analysis by McGaugh and his colleagues of DF2’s implications for MOND hinges on the galaxy’s proximity to the massive elliptical galaxy NGC1052. Under a set of “reasonable” assumptions, paired with equations from MOND, McGaugh and his colleagues found that NGC1052’s gravitational effects on DF2 would return stellar velocities similar to what van Dokkum and Danieli actually observed. Although he hasn’t had the chance to repeat this analysis for DF4, McGaugh says it also “appears to be consistent with MOND, since it is likely affected by NGC 1052.”

The existence of these galaxies poses a number of vexing problems for the theory of galactic formation, which must account for how a galaxy can come to be violently stripped of its dark matter and still retain the relative order seen, for example, in the presence of star clusters in DF2 and DF4. Will further observations of ultra-diffuse galaxies resolve the dark matter debate? Probably not, but they will, at least, shed some light on the matter.


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